1. Field of the Invention
The present invention relates to a fluidized bed reactor, and more specifically to a fluidized bed reactor which is suitable to treat particles or gas which change properties by the reaction, and which includes a reactor vessel which is vertically divided into three regions by spaced distributor plates, a gas inlet conduit through the lower region, a particles inlet which introduces particles to be treated into the middle region, a product gas outlet communicated with the upper region, and an overflow which discharges reacted particles from the middle region.
2. Description of the Prior Art
Recently, after successful results obtained by fluid catalytic cracking methods in the petroleum industry, fluidized bed reactors are being broadly utilized in the chemical and metallurgical industries as catalytic reaction, calcination, drying and particle transportation apparatus. The fluidized bed reactor is a reaction device which forms a fluidized bed formed by the reaction particles by blowing fluidizing gas into a fixed bed of the reaction particles.
Generally, transformation from a fixed bed to a fluidized bed is illustrated in FIG. 1. The fluid velocity is illustrated by the logarithmic scale in FIG. 1. In FIG. 1, range (a) is a fixed bed and the second range (b) is a fluidized bed. In the fixed bed, pressure drop in the bed increases as fluid velocity is increased, i.e. as fluid flow rate increases. That is, particles float in fluid flow by fluid resistance applied to the particles against gravitational force. At a fluid velocity, the pressure drop is constant. The floating condition of the particles is called fluidized bed. In this specification, the critical fluid velocity corresponding to the transition from the fixed bed to the fluidized bed is called minimum fluidizing velocity Umf, which varies depending upon the properties of the particles, e.g. diameter, specific gravity and sphericity of particles.
Fluidized bed reactors utilize the above mentioned characteristics of a fluidized bed. It is desired to maintain good fluidization, i.e. movement of the particles is excellent and entrainment of the particles is less. To obtain such fluidization condition, it is necessary to control the fluid velocity U of gas which has passed through the fluidizing bed. As the properties of the fluid and reacting particles change by the reaction process, the minimum fluidizing velocity Umf is also changed. Thus, the velocity Umf must be detected to obtain proper control. Thus, it is desirable to monitor the fluid velocity U and the minimum fluidizing velocity Umf continuously all through the operation, and also it is desirable to obtain the function f (U, Umf) in a form which can be utilized to operate the fluidized bed.
Methods to detect the velocities U and Umf which have been proposed are as follows:
(1) Means to assume the velocity Umf, by the sampling method.
Property of particles under reaction, e.g. diameter, density and sphericity of particles, and property of fluid under reaction, e.g. viscosity and density, are detected by sampling and analysis, and the velocity Umf is assumed. The sampling and analysis necessitate relatively long time so that it is difficult to obtain continuous data to be used as operation control. When the properties are not detected, accurate assumed value of the velocity Umf cannot be obtained. Experimental formulae and theoretical formulae to assume the velocity Umf are not accurate enough, especially at a high temperature range.
(2) Means to assume the fluid velocity U by the fluid velocity measuring method.
The fluid velocity is measured directly or indirectly outside the reactor. Indirect measurement includes the disadvantage that the vaporizable liquids content must be added afterwards. Reliability of the measurements is very low, as, many factor, e.g. pressure, temperature, particle entrainment, influence the fluid velocity under operation.
(3) Method to assume fluid velocity U by the gas quantity measuring method.
When gas is obtained as a product of the reactor, the produced gas is guided outside the reactor and the gas quantity is measured. From the gas quantity, minimum fluidizing velocity Umf is assumed. Normally, steam and vapor are condensed before the measurement. Thus, it is not easy to assume true fluid flow condition from such measurement of dry gas.
As stated above, conventional measurement methods measure the fluid velocity U and the minimum fluidizing velocity Umf independently, and no reliable result can be obtained. Relation between the method of operating the reactor and the velocities U and Umf is as follows: The operating condition of the fluidized bed reactor is determined based on the velocities U and Umf, and the reactor is actually operated by the determined operating condition. Actual velocities U and Umf are measured under operation, and the operating condition is modified. Such operating method is suitable for an established process. However, when it is desirable to introduce a new operating condition, some means is necessary to judge whether factors to be controlled do or do not coincide with the predetermined or expected values.
Methods of the judgement which have been proposed are as follows:
(4) Method of measuring the pressure drop across a fluidized bed.
The pressure drop across a fluidized bed is one of the most suitable measurable factors to judge the operating condition of a fluidized bed reactor as the pressure drop relates directly to the motion of particles. However, the pressure drop cannot be quantitatively related to all operating conditions of fluidized bed reactors. Thus, under normal operating conditions of a specified reactor for an established process, the pressure drop can be successfully utilized to assist experimental judgement. In transient operating conditions, e.g. starting up, or in new operation, monitoring the pressure drop cannot maintain a suitable fluidized bed condition.
(5) Method of measuring temperature distribution in fluidized bed.
Temperatures are measured at many points in the reactor vertically and horizontally, to know operating condition of the fluidized bed. Generally, when particle movement is strong, reactor temperature is substantially uniform. When any stagnant zone is produced, local temperature change indicates where the stagnant zone is. Uniformity of the temperature directly relates to the fluid velocity U, and temperature difference across the fluidized bed decreases as the fluid velocity is increased. However, it is not necessary to increase the fluid velocity more than needed to maintain the fluidized bed. The temperature distribution cannot quantitatively be related to all operating conditions of the fluidized bed, so that is is also used to assist in the experimental decision under normal operating conditions. As described in detail, conventional operation of fluidized bed reactors has been performed experimentally with assistance of pressure drop across fluidized bed and temperature distribution, and also inaccurate assumed values of fluid velocity U and minimum fluidizing velocity Umf. As the velocities U and Umf cannot accurately be obtained, safe side operation or erroneous operation may be the result. This means disturbance to the development of fluidized bed operating technics.